Filamentous fungi are capable of producing high amounts of extracellular proteins. Yet, the production level of any protein of interest in naturally occurring strains is usually too low for commercial exploitation, rendering substantial strain improvement programs essential (Punt 2002). In industrial filamentous fungi, this is traditionally done by classical mutagenesis, and/or targeted gene manipulation in combination with protein engineering. Basically, this approach consists of subjecting the fungus to sub-lethal doses of mutagens (frequently used: for example irradiation by UV light or ionizing radiation, addition of chemical mutagens like nitrosomethyl guanidine, ethylmethansulfonat or ethidium bromide) and subsequent screening of the survivors for improved production of the desired product. Obviously, this approach is only as good as the selection method.
Trichoderma reesei (teleomorph Hypocrea jecorina) is a workhorse organism for the industrial production of enzymes. Using random mutagenesis, academic and industrial research programs have, over several decades, produced strains of T. reesei whose enzyme productivity is several times higher than that of the “original” T. reesei strain QM6a that was isolated from US Army tent canvas in 1944 in the Solomon Islands (LeCrom et al. 2009).
A major constraint using random mutagenesis for strain improvement, however, results from the fact that, by definition it cannot be directed to act on distinct target genes. Mutations may thus not only be beneficial and improve or disrupt target genes but can affect other genes as well leading to unwanted collateral damage causing restrictions in e.g. strain stability, reduced growth rates or auxotrophy for amino acids and/or vitamins. While recombinant techniques overcome these problems by introducing targeting focus, they may be unsuitable for complex genetic traits that are caused by unknown or multiple genes or large genomic fragments.
Seidl et al. (2009) were the first to describe the ability of Trichoderma reesei to perform sexual reproduction. Taxonomically T. reesei (and its teleomorph H. jecorina) belong to the group of Ascomycetes (class of Sordariomycetes) and within this group to those fungi which are heterothallic. Heterothallism means that the two mating type loci MAT1-1 and MAT1-2 which are necessary for successful sexual reproduction occur in different strains and self-fertilization is not possible.
During the process of sexual reproduction T. reesei produces perithecia that are initiated by coiled or variously distinctive structures containing the ascogonium. The ascogonium is the cell that will receive a nucleus during mating and go on to produce the dikaryotic hyphal system. One of the cells within the coil functions as the ascogonium and the rest either remains inactive or gives rise to hyphae that branch and proliferate to surround the entire structure. In many cases neighboring hyphae also engage to envelope the coil. These surrounding hyphae eventually consolidate to form the walls or peridium of the perithecium (http://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/SordariomycetesDiscussion.html) (FIG. 3).
The formation of perithecia in T. reesei occurs ascohymenial meaning that the ascocarp formation initiates with the fertilization of the ascogonium. The primordium subsequently differentiates into a true hymenium directly from the generative, ascogeneous hyphae. The ascohymenial development thus begins with fertilization and differentiation of generative hyphae, followed by the development of the ascocarp. Stromata (fruiting bodies) are therefore formed by the partner acting as a female during sexual reproduction.
Since all strains of T. reesei which are nowadays used in industry can be traced back to strain QM6a they all carry—as does strain QM6a—the MAT1-2 locus. Crossing of different industrial strains with each other to further improve them by introducing favorable traits or to rid strains from mutations of genes or undesired genes such as e.g. genes conferring e.g. resistance to antibiotics, coding undesired products whose presence may interfere with regulatory requirements is therefore not possible at present.
A possibility to overcome the inability to cross different T. reesei mutant strains would be to exchange the mating type locus by the opposite one at the same genomic locus. In case of T. reesei QM6a this implies to exchange the MAT1-2 locus for the MAT1-1 locus.
Kang et al. (1994) have shown that strains of Magnaporthe grisea in which the MAT locus was exchanged were fertile in crossings with strains of the opposite mating type (i.e.: strains in which the MAT1-2 locus was replaced by the MAT1-1 locus were fertile in crossings with the original MAT1-2 carrying strain). A successful exchange of the mating type locus was also described for Neurospora crassa (Chang, 1994).
WO 2011/095374 relates to the use of mating type switching to improve the sexual behavior of filamentous fungal strains. Disclosed is the identification of mating types of Aspergillus niger and Aspergillus tubigensis so as to transform Aspergillus niger into a heterothallic fungus, i.e. filamentous fungus individuals having opposite mating types resulting in one or more pair of strains with two opposite mating types.
Seidl et al. (2009) introduced the complementary mating type locus (MAT1-1) ectopically into T. reesei strain QM6a thereby generating a strain carrying both mating type loci (MAT1-1 and MAT1-2). This strain was fertile in crossings with wild type strains of the T. reesei teleomorph H. jecorina carrying either the MAT1-1 or the MAT1-2 locus. However, in crossings with strain QM6a and its derivatives (all MAT1-2) this strain was found to be sterile.
From these results—that a QM6a strain carrying both mating types is able to form fruiting bodies with MAT1-1 and MAT1-2 strains of H. jecorina, but not with QM6a—it is concluded that T. reesei QM6a is able to act as a male partner but that it cannot produce fruiting bodies and is therefore female sterile. Probably its maintenance in the labs for over 60 years without selective pressure acting to maintain mating competence has resulted in mutations in one or more of the genes necessary for sexual recombination.
Hence, there is a need in the prior art for a method that allows the rapid and efficient identification of genes associated with said mating impairment of a Trichoderma reesei QM6a strain. So far, the genes associated with mating impairment in the genome of Trichoderma reesei QM6a have neither been identified nor been characterized. The reason for this is mainly due to the fact that classical genetic approaches using sexual crossings have not been established for Trichoderma reesei QM6a due to its self-sterility. It is, moreover, not known which genes contribute to or account for the self-sterility of QM6a. So far, it is completely unknown which of the 9143 annotated genes in the genome of QM6a (34.1 Mbp) is associated with its self-sterility. The genome of T. reesei QM6a has been published in Martinez et al. (2008). The T. reesei nucleotide sequence and annotation data have been deposited in GenBank under accession number AAIL 00000000.
The present inventors have recently topically replaced the MAT1-2 locus of QM6a with the opposite mating type (MAT1-1) at the same genomic location, but crossings of the resulting MAT1-1 strains with T. reesei QM6a derived MAT1-2 strains were not successful demonstrating self-sterility.
It is, therefore, an object of the invention to identify and provide genes/genetic elements associated with the mating impairment in strains of T. reesei QM6a and its derivatives.
It is further an object of the invention to provide a process for the rapid and efficient identification of genes associated with mating impairment in T. reesei QM6a. It is a further object of the invention to provide a process for restoring the mating competence of Trichoderma reesei QM6a and its derivatives. It is a further object of the invention to provide a mating competent form of T. reesei QM6a and its derivatives. It is a further object of the invention to provide a process to sexually recombine genetic information of Trichoderma reesei QM6a and its derivatives. Another object of the invention is to prepare a strain of Trichoderma reesei QM6a or of a derivative thereof having a sexual cycle.
The inventors have surprisingly found that the mating impairment of a Trichoderma reesei strain QM6a or a strain derived thereof is caused by defined genes/genetic elements and mutations in defined genes/genetic elements of said organism and may be corrected by correcting or eliminating said mutations, i.e. by replacing the corresponding gene(s)/genetic element(s) with a functional counterpart or by inserting the gene/genetic element that is completely or partially missing. A functional counterpart of said gene(s)/genetic element(s) is a gene or genetic element that restores the mating ability of Trichoderma reesei QM6a. Said capability is conferred to said gene/genetic element by correcting the mutations as depicted in Table 3 to such an extent that the gene or genetic element performs its function in the mating process. Preferably all mutations of the respective gene/genetic element are corrected.
It has further been found that for a Trichoderma reesei QM6a strain or any derivative thereof the technique of backcrossing may advantageously be used for identifying the genes or genetic elements that are associated with mating impairment in Trichoderma reesei QM6a or any derivative thereof (female sterility genes). Moreover, it has been found that mutations in certain genes are associated with mating impairment in Trichoderma reesei QM6a. The mutation may be a simple point mutation, an insertion or a deletion, whereby the deletion may be a deletion within an existing gene or a full or partial deletion of a gene/genetic element per se.
The above genes or genetic elements/genetic information may be contributory to mating impairment or may cause—singly or in combination—mating impairment in Trichoderma reesei QM6a. Hence, said genes/genetic elements/genetic information may be directly or indirectly associated with the mating impairment of said organism.
Genes or genetic elements/genetic information directly associated with the mating impairment of Trichoderma reesei QM6a and its derivatives are generally genes of which the full or partial lack of functioning leads to a reduced or complete lack of formation of fruiting bodies when crossed with a corresponding organism. The term “mating impairment” is to relate to all degrees of an impaired or reduced mating ability. A reduced mating ability can thus also be seen in a substantially prolonged time until mature fruiting bodies with viable ascospores become visible. Corresponding genes and/or genetic elements have a direct impact on any organ or metabolic mechanism that has a direct impact on the mating ability of Trichoderma reesei QM6a and its derivatives, such as for example genes coding for pheromone receptors or genes coding for organs needed for successful mating. A genetic element comprises genetic information that is not translated into a protein but is directly or indirectly related to the mating behaviour of Trichoderma reesei QM6a or its derivative. Said genetic information may be necessary for the control or regulation of the expression of a protein. A genetic element may be a promoter, an enhancer, an activator, a regulator or an expression control sequence.
Genes/genetic elements indirectly associated with the mating impairment of Trichoderma reesei QM6a and its derivatives are genes which are not directly associated with the mating impairment of said organism but which relate to morphological structures or metabolic mechanisms that have an indirect impact on the ability of Trichoderma reesei QM6a and its derivatives to mate with a corresponding organism. Mutations in genes for example related to hyphal cell wall structure or hyphal branching could have an indirect impact on the mating ability of Trichoderma reesei QM6a and its derivatives. The function of said genes may be executed by proteins encoded by said genes. Said gene/genetic element may also be necessary for expression or regulation of a corresponding protein (p. ex. promoter, enhancer, activator, regulator, initiator, expression control sequence).